The posttranslational attachment of ubiquitin to proteins is used in eukaryotes to target a broad range of proteins for selective degradation. Selective removal of proteins is required for a number of cellular processes and a requirement for ubiquitin has been shown for processes ranging from the determination of cell fate in division, differentiation, oncogenesis and senescence to more specific processes such as DNA repair, inflammatory response, signal transduction and the removal of abnormal proteins. Many of these processes are also under intense investigation because of their relevance in human diseases. Oncogenesis and the inflammatory responses are two examples. The specificity of protein ubiquitination is conferred by a family of related proteins known as ubiquitin conjugating-enzymes (Ubc). Members in this family interact with a distinct set of substrate-binding proteins (also known as E3), and it is these cascades of interaction between Ubc and E3 proteins that enable the pathway to target a broad range of proteins for degradation. Ubc proteins all share a common conserved domain of approximately 150-170 amino acid residues. The Class I enzymes are the smallest and are comprised entirely of this domain. This conserved domain in larger, multifunctional Ubc's can also function autonomously. Thus, there must be unique structural features embedded in the conserved domain of an individual Ubc protein that enable each enzyme to interact specifically with its own set of E3 proteins. Analyses of Ubc sequences and the three known crystal structures of Class I enzymes have led us to formulate a general model that predicts how similarity and differences in Ubc's are manifested structurally. Our long term goal is to determine how specific regions in Ubc proteins are utilized for its function. The specific aims for this research period is 1) to analyze the effect of mutations on the function of selected Ubc's in the yeast S. cerevisiae, with the intention of defining those regions that are responsible for Ubc interaction with E3, 2)to initiate studies toward determining the crystal structure of a Ubc4-E3 complex; 3) to define E3 proteins that function in the Ubc7-dependent pathway.